A plain bearing, as contrasted with a rolling bearing has a sliding surface with low wearing and low slding friction characteristics
In general, plain bearings include a support or base upon which a sliding layer is applied either directly or indirectly to form the bearing face.
In such bearings, wear of the sliding layer which is caused by abrasion may lead to damage to the bearing as well as of the rotating element, e.g. a shaft journaled in the bearing bushing or sleeve
Since the wear of the sliding layer causes also an increase of temperature in the area of the bearing face in comparison with the normal operational temperature, preventive measures are required which can include monitoring of the temperature of the sliding layer. Thus, bearing damages and resulting extensive repair costs can be avoided when a temperature increase is detected in time.
U.S. Pat. No. 2,961,875 describes a device for sensing the temperature of bearings which includes a thermocouple having a pair of wires which extend within a borehole provided in the support base of the bearing. The borehole is accessible from outside and leads to the sliding layer. At the end adjacent to the sliding layer, the wires are inserted in a plug after being joined together and placed in direct heat transfer relationship with the sliding layer and the interface between this layer and the rotary member.
The publication "messen +prufen/Automatik, July/August 1979, pages 567 to 573, describes thermoelements inserted in such boreholes which can be fixed via inserted plugs, or by completely filling the borehole with the thermoelement. In the latter case, the thermoelement is made of a solid material having the same temperature and heat transfer coefficients as the material for the bearing.
These systems have the disadvantage that a replacement of the plain bearing will either render a reuse of the but in thermoelements impossible or the retrieval of the thermoelements very complicated and expensive .
The German patent document DE-GM No. 73 13 958 thus proposes to provide the bores within the support base of the sliding bearing and in the bearing nest of the bearing housing in such a manner that a temperature sensor can be inserted and removed at any time. Since vibration or other relative movement between the sliding bearing and the bearing nest cannot be prevented, the bores must be provided with such a dimension that a sufficient play of the temperature sensor is dotained within the bore. On the other hand, it must be taken into account that the provision of play does not allow a fixed and stable attachment of the temperature sensor to the support base. Such an attachment, however, is necessary for providing a reliable determination of temperature. A further drawback of this construction is that once the slding layer wears out, the temperature sensors as inserted according to the DE-GM No. 73 13 958 can come into direct contact with the surface of the machine part supported by the sliding bearing, and thus would not only impair the operation of the bearing but would also be damaged. Thus, this system does not provide a reliable determination of the temperature of the bearing running surface upon gradually increasing wear of the sliding layer
In order to avoid these deficiencies caused when inserting temperature sensors in boreholes, the German Democratic Republic patent DD-PS No. 139 299 proposes to use the sliding layer and the support base of the sliding bearing as parts of a thermoelement since these parts are made of different materials and thus can be expected to have a defined thermo-electromotive force. This technique has, however, also considerable drawbacks. Apart from the fact that the provision of electric contacts with the sliding layer of the bearings is difficult and cumbersome to achieve and such contacts will be damaged once the wear of the sliding layer increases, it should be noted that due to the electrical conductivity of the sliding layer and the support base only an average temperature of the entire bearing face can be determined. A local measurement of the temperature cannot be provided. Further this prior art requires a direct connection between the sliding layer and the support base and thus is not possible when thin layers are used for different functions between the support base and the sliding layer.